A particulate filter (hereinafter simply referred to as a “filter”) may be placed in an exhaust passage of an internal combustion engine to trap particulate matter (hereinafter referred to as “PM”) in the exhaust gas of the internal combustion engine. In this case, operation of the internal combustion engine may be negatively affected by increase of the back-pressure, if the PM accumulation amount at the filter should increase excessively. Further, if the PM accumulates excessively at the filter and is oxidized, the filter temperature may increase excessively by the oxidation heat, which may eventually cause erosion or breakage of the filter. To prevent such problems, a filter regeneration system conducting a regeneration process by oxidizing and removing the PM accumulated at the filter has been employed.
Patent Document 1 describes a technology concerning an exhaust gas purification device carrying out a regeneration process as described above. The exhaust gas purification device disclosed in Patent Document 1 is provided with the first decision means for deciding the timing of the regeneration process, when the differential pressure across the filter exceeds a pre-determined value, and the second decision means for deciding the timing of the regeneration process, when the PM accumulation amount reaches or exceeds a pre-determined value. Further, a number of consecutive regenerations, namely series of busy regeneration periods whose regeneration time intervals between the decisions for the regeneration process are shorter than a threshold value, is counted, and if the number exceeds a threshold value, the PM discharged amount from the engine is judged abnormal.    [Patent Document 1] Japanese Patent No. 4008866    [Patent Document 2] Japanese Patent Laid-Open No. 2008-57443    [Patent Document 3] Japanese Patent Laid-Open No. 2008-121631    [Patent Document 4] Japanese Patent Laid-Open No. 2004-218558